1. Field of the Invention
This invention relates to a porous or non-porous substrate comprising a fluorine-containing polymer including a polyperfluorocarbon substrate having a surface with improved hydrophilic characteristics, comprising a branched fluorocarbon polymeric composition having hydrophilic functional groups or a crosslinked fluorocarbon polymeric composition having hydrophilic functional groups and to a process for forming the substrate having the improved hydrophilic characteristics. More particularly, this invention relates to a porous or non-porous substrate comprising a fluorine-containing polymer including a polyperfluorocarbon substrate having a surface with improved hydrophilic characteristics, comprising a branched fluorocarbon polymeric composition having hydrophilic functional groups or a crosslinked fluorocarbon polymeric composition having hydrophilic functional groups wherein the functional groups comprise sulfonyl methide, sulfonyl methane, amide, sulfonamide, imide or sulfonimide groups and to a process for forming the substrate having the improved hydrophilic characteristics
2. Description of the Prior Art
Articles made of a fluorine-containing polymer including a polyperfluorocarbon substrate are useful in a wide variety of environments due to the chemical inertness of the substrate. By the term “polyperfluorocarbon” as used herein is meant a polymer comprising one or more perfluorocarbon monomers including homopolymers, copolymers, terpolymers, polymer blends or the like. Examples of polyperfluorocarbons include polytetrafluoroethylene, fluorinated ethylene-propylene copolymer (FEP) and perfluoroalkoxy polymer (PFA). These substrates are formed into a variety of shapes including non-porous films, beads, tubes, woven fibers, non-woven fibers, porous membranes, or the like.
Porous membrane filters are utilized in a wide variety of environments to separate materials within a fluid stream. Membranes are formed from a solid polymeric matrix and have highly precisely controlled and measurable porosity, pore size and thickness. In use, the membrane filters generally are incorporated into a device such as a cartridge, which in turn, is adapted to be inserted within a fluid stream to effect removal of particles, microorganisms or a solute from liquids and gases.
To be useful, membrane filters must be resistant to the fluid being filtered so that they maintains their strength, porosity, chemical integrity and cleanliness. In addition membrane filters must remain wetted with the process fluid in order to retain filtration efficiency. Polyperfluorocarbon-based membrane filters made from fluorine-containing polymers such as polytetrafluoroethylene, FEP or PFA are commonly utilized in these applications. Fluorine-containing polymers are well known for their chemical inertness, or excellent resistance to chemical attack.
One disadvantage of fluorine-containing polymers is that they are hydrophobic and therefore membranes made from such polymers are difficult to wet or to remain wet with aqueous fluids or other fluids which have a tendency to outgas. For example, in the manufacture of microelectronic circuits, membrane filters are used extensively to purify various outgassing process fluids to prevent contaminants from causing circuit failures. Thus it would be desirable to provide a membrane filter having improved hydrophilic properties, which does not dewet during filtration in order to retain filtration efficiency.
Dewetting of a fluorine-containing polymeric surface also is a problem with a non-porous surface such as a hollow tube wherein the formation of gases therein restricts or prevents the flow of an outgassing liquid therein.
When modifying a porous membrane surface for filtration applications, it is essential that the surface modification to impart improved hydrophilic characteristics be effected while maintaining desired fluid flow properties through the modified membrane. Thus, the membrane having its surface modified should retain sufficient porosity to permit its use as a filtration member. Accordingly the amount of surface modifying composition applied to the membrane surface must be controlled in order to retain porosity of the modified membrane. In contrast, when modifying a porous membrane for non-filtration applications such as in ion-exchange applications including electrodialysis, electrodeionization or the like, complete blockage of the membrane pores with the surface modifying composition is not a problem.
It has been proposed and U.S. Pat. No. 5,928,792 which is incorporated herein by reference to provide a process for modifying a surface of a porous membrane such as a perfluorocarbon membrane with a bound perfluorocarbon copolymer composition to render the entire surface non-dewetting.
PCT application WO 99/38897 discloses crosslinked sulfonated polymers and the process for preparing them.
U.S. Pat. No. 4,470,859 to Benezra et al, discloses a process for modifying the surfaces of microporous substrates formed of a fluorocarbon such as polytetrafluoroethylene, with a coating of an uncrosslinked perfluorocarbon copolymer composition including hydrophilic functional groups copolymer from a solution of the pefluorocarbon copolymer composition to render the surface of the membrane more water wettable. The perfluorocarbon copolymer composition is dissolved in a nonaqueous solvent at elevated temperature. The membrane then is immersed into the solution which, in turn, is placed into a vacuum chamber. The pressure within the chamber then is reduced such as to approximately 150 millimeters of mercury (absolute) to remove air from within the filter. Thereafter, the pressure within the chamber is quickly returned to atmospheric pressure. This coating process is repeated to ensure, what is described by Benezra et al., complete solution penetration into the pores of the membrane. By proceeding in this manner, the membrane surfaces and the interior walls defining the interstices within the membrane are coated with the perfluorocarbon copolymer composition. Following the coating step, the solvent is removed by evaporation using heat and vacuum, or the solvated perfluorocarbon copolymer composition is precipitated with a substance in which the perfluorocarbon copolymer composition is effectively insoluble. The solvents utilized to form the solution include halocarbon oil, perfluorooctanoic acid, decafluorobiphenyl, N-butylacetamide, and N,N-dimethylacetamide. Subsequent to modifying the membrane surface, Benezra et al, teaches avoiding the use of a fluid containing a solvent for the modifying perfluorocarbon polymer composition on the membrane surface. Benezra et al. also disclose that alcohol solutions for the perfluorocarbon polymer composition should be avoided.
U.S. Pat. No. 4,902,308 to Mallouk et al, also describes a process for modifying the surface of a porous, expanded polytetrafluoroethylene membrane with a perfluoro-cation exchange polymer from an organic solution of the polymer. Mallouk et al, also teaches that contact of the surface modified membrane with fluids containing a solvent for the polymer also should be avoided.
U.S. Pat. Nos. 4,259,226 and 4,327,010 disclose modifying a porous membrane surface with a fluorinated polymer having carboxylic acid salt groups. No process steps are disclosed for controlling extractables from the membrane or for controlling the extent of binding of the modifying composition to the membrane surface.
U.S. Pat. Nos. 5,183,545 and 5,094,895 disclose a process for making a multilayer, composite, porous diaphragm lrom a porous, multilayer, expanded polytetrafluoroethylene substrate having its surface modlified with an uncrosslinked perfluoro ion exchange polymer composition. The modifying polymer composition can contain a surfactant and may contain excess modifying composition, both of which are sources of undesirable extractables.
U.S. Pat. No. 5,874,616 to Howells et al discloses a process for making linear polymeric bis(fluoroalkylenesulfonyl)imides by reacting a difunctional fluoroalkylene sulfonamide compound with a difunctional fluoroalkylene sulfonyl halide compound. Since this process relies on the use of low molecular weight reactants to produce the polymer, it is difficult to control the molecular weight distribution of the final linear polymeric composition and therefore it is undesirable. In addition, the linear polymeric bis(fluoroalkylenesulfonyl)imides cannot be crosslinked.
U.S. Pat. No. 5,463,005 to Desmarteau discloses a process for forming a copolymer of tetrafluoroethylene (TFE) and a sulfonimide-containing unsaturated monomer wherein the monomer is derived from a sulfonimide-containing reagent. Since TFE is a toxic reagent, its use is undesirable. In addition, since this process relies on the use of low molecular weight unsaturated monomers, it is difficult to control the molecular weight distribution of the final polymeric composition.
It has been proposed by Shimazo in J. Electroanal. Chem. 258 (1998) pp. 49–59 to crosslink a perfluorocarbon copolymer composition such as Nafion™ films with radio frequency plasma. Crosslinking with a plasma is effected without the introduction of crosslinking groups. Crosslinking is effected through a bond linking two polymer chains. Unfortunately, ionizing radiation such as plasma degrades the perfluorocarbon copolymer composition.
It has been proposed by Greso et al in POLYMER vol. 38 No. 6 (1997), pp. 1345–1356 to crosslink a perfluorocarbon copolymer composition such as Nafion™ films via Si—O—Si bridges. This process is undesirable since the product contains a bound inorganic phase, which is chemically unstable.
It has been proposed by Covitch et al in Polymer Science and Technology Vol. 16, pp. 257–267 (1982) to crosslink a perfluorocarbon copolymer composition such as Nafion™ films with ethylenediamine and heat. This process is undesirable since the product contains a non-fluorinated ethylene portion, which is chemically unstable.
Accordingly, it would be desirable to provide a porous membrane formed from a fluorine-containing polymer having a surface with improved hydrophilic characteristics suitable for filtration applications. Such a porous membrane would be chemically and thermally stable and have a surface sufficiently hydrophilic to enable filtration while avoiding dewetting. In addition, it would be desirable to provide a non-porous membrane formed from an initially porous membrane formed from a fluorine-containing polymer having its pores filled with a fluorocarbon polymeric composition having hydrophilic functional groups. Such a non-porous membrane would be chemically and thermally stable and would permit the transport of ionic species therethrough. Furthermore, it would be desirable to provide a surface modified article formed from a non-porous fluorine-containing polymeric substrate having its surface modified with a fluorocarbon polymeric composition having hydrophilic functional groups. Such a non-porous article would be useful for processing outgassing liquids while avoiding dewetting of the surface.